The present invention relates to a chip positive temperature coefficient (hereinafter, PTC) thermistor comprising conductive polymers having PTC properties.
PTC thermistors can be used as an overcurrent protection element. When overcurrent is applied in an electric circuit, conductive polymers with PTC properties spontaneously emit heat and thermally expand to become a high resistance, thereby lowering the current to a safe small-current level.
The following is a description of a conventional PTC thermistor.
The Japanese Patent Laid-Open Publication No. S61-10203 has disclosed a PTC thermistor. This conventional PTC thermistor is constructed such that, a plurality of conductive polymer sheets and metal foils are alternately laminated and drawing-out sections are provided on opposite side faces.
FIG. 21 is a cross section of a conventional PTC thermistor.
FIG. 21 shows conductive polymer sheets 1a, 1b and 1c made with cross-linked polymer materials such as polyethylene in which conductive particles such as carbon black is mixed. Electrodes 2a, 2b, 2c and 2d made with copper or nickel, sandwich the conductive polymer sheets 1a, 1b and 1c in between them in such a manner that openings 3 are formed alternately on opposite edges of the electrodes 2a, 2b, 2c and 2d. By layering these electrodes 2a, 2b, 2c and 2d, and the conductive polymer sheet 1a, 1b and 1c alternately, a laminate 4 is formed. On side faces of drawing-out sections 5a and 5b which are electrically coupled with one-end of each of the electrodes 2a, 2b, 2c and 2d. 
However, in the case of the construction of the conventional PTC thermistor, due to the considerably large differences in thermal expansion coefficients between the component materials: the thermal expansion coefficient of copper is 1.62 xc3x9710xe2x88x925/K; nickel, 5.3 xc3x9710xe2x88x925/K; and polyethylene, approximately 5 xc3x9710xe2x88x924/K which increases by 1 digit at and over 130xc2x0 C., mechanical stress during operation of the PTC thermistor has caused cracks, degrading the-connection points between the electrodes 2a, 2c and the drawing-out section 5a, and between the electrodes 2b, 2d and the drawing-out section 5b, and in some cases, triggered breaking of the connection.
Furthermore, since drawing-out section 5a does not extend to the lowest point of the conductive polymer sheet 1c, it has failed to be used as a component for a surface mount assembly.
To address these issues, the drawing-out section 5a of another conventional chip PTC thermistor has been extended to the lowest point of the conductive polymer sheet 1c so that it can be mounted on a surface. FIGS. 22 (a), (b) and (c) show cross sections of such PTC thermistor respectively with a single laminate, a double laminate, and a triple laminate. FIG. 23 is a cross section in which single laminate is mounted on lands 6a and 6b with solder 7a and 7b. However, even with this construction which has surface mountability, connecting point 9 of the electrode 2a and the drawing-out section 5a has suffered degradation due to cracks, and in some cases, breaking of the connection.
The present invention aims to address the foregoing problems by providing a chip PTC thermistor which achieves a superior long-term connection reliability between electrodes and drawing-out sections, namely side electrodes, and is applicable to surface mounting.
To achieve the foregoing objects, the Chip PTC thermistor of the present invention comprises;
a) a rectangular parallelepiped conductive polymer having PTC properties;
b) a first main electrode disposed on a first face of the conductive polymer;
c) a first sub electrode disposed independently, but on the same face as the first electrode;
d) a second main electrode disposed on a second face opposite the first face of the conductive polymer;
e) a second sub electrode disposed independently, but on the same face as the second main electrode;
f) a first side electrode folding around and over the entire surface of one of side faces of the conductive polymer, the first side electrode electrically coupling the first main electrode and the second sub electrode; and
g) a second side electrode folding around and over the entire surface of the other side face, opposite the side face mentioned previously, of the conductive polymer, the second side electrode electrically coupling the first sub electrode and the second main electrode.
Wherein the first and second side electrodes comprise a nickel plating layer or a nickel alloy plating layer whose thickness is not less than one twentieth of the distance between the first and second main electrodes.
According to the foregoing construction of the chip PTC thermistor, the side electrodes fold around to cover not only the entire surface on the side of the conductive polymer but also to cover end surfaces of the main and sub electrodes. As such, the side electrodes and the main and sub electrodes are not linearly coupled but coupled with faces. Furthermore, the side electrodes are made with nickel which has high mechanical strength and formed with a plating layer of an even thickness equal to or thicker than one twentieth of the distance between the first and second main electrodes. Thus, the PTC thermistor of the present invention is surface mountable and shows a superior long-term connection reliability between the main and sub electrodes and the side electrodes.